1. Field of the Invention
The present invention relates to a semiconductor laser. In particular, the present invention relates to a semiconductor laser whose polarization mode of output light is switchable. Further, the present invention relates to a semiconductor laser array including a plurality of lasers, an optical transmitter using the semiconductor laser, an optical communication system using the semiconductor laser and a fabrication method of the semiconductor laser.
2. Related Background Art
In recent years, optical communication technologies using optical devices, such as a semiconductor laser, have solved a number of problems. However, a problem of so-called chirping has not yet been solved. The chirping is a phenomenon that the waveform of output light from a semiconductor laser is distorted by fluctuation in a refractive index due to carriers' uneven distribution caused at the time of a high-speed intensity modulation of the semiconductor laser. A primary method for solving the chirping, which is presently employed, is an external modulation method in which a semiconductor laser is driven in a CW (continuous wave) manner and its output light is intensity-modulated by an external semiconductor optical modulator. Recent studies, however, revealed that there is still a limitation to reduction of the chirping obtained by that external modulation method.
In contrast therewith, in a polarization selective or switchable laser for switching its polarization plane of laser light in accordance with a signal, the chirping is small and its modulation speed and transmission distance can be improved, compared with an ordinary intensity-modulation laser, since it is possible for the polarization selective laser to keep the light density and the carrier density in its cavity almost constant even during its modulation time. With respect to the polarization selective laser, Japanese Patent Publication No. 5-68111 and Japanese Patent Laid-Open Application No. 7-235718 disclose semiconductor laser apparatuses, respectively. In the semiconductor laser apparatus, there are arranged two semiconductor lasers connected serially or in parallel, and one of them chiefly generates or amplifies light in a predetermined polarization state (a transverse electric (TE) mode or a transverse magnetic (TM) mode) while the other chiefly generates or amplifies light in another polarization state (the TE mode or the TM mode). Further, light waves generated or amplified by one semiconductor laser can be superimposed on light waves generated or amplified by the other.
FIG. 30 illustrates the schematic structure of a conventional semiconductor laser apparatus. The semiconductor laser apparatus is comprised of two different semiconductor lasers connected serially. Its first semiconductor laser includes a gain region 1103a in which a gain for the TE mode always exceeds a gain for the TM mode, its second semiconductor laser includes a gain region 1103b which has gain characteristics opposite to those of the first semiconductor laser, and the respective semiconductor lasers have distributed reflectors 1107a and 1107b, such as gratings or the like whose Bragg wavelengths are respectively set close to peak wavelengths of gain spectra of the first and second semiconductor lasers. Its operation principle is as follows: Carriers are independently injected into the first and second semiconductor lasers, and the gains for the TE mode and the TM mode are brought to their equal threshold gain. With using this state as a bias point, the polarization switching between the TE mode and the TM mode is conducted by slightly modulating carriers injected into at least one of the two semiconductor lasers. Compared with an ordinary intensity-modulation laser, the polarization selective or switchable laser is said to be superior thereto in that the chirping is small and its modulation speed and transmission distance can be improved since it is possible for the polarization selective laser to keep the light density and the carrier density in its cavity almost constant even during its modulation time, as mentioned above. In FIG. 30, reference numeral 1101 denotes a substrate. Reference numeral 1102 denotes a lower clad layer. Reference numeral 1104 denotes a light guide layer. Reference numeral 1105 denotes an upper clad layer. Reference numeral 1106 denotes a contact layer. Reference numerals 1109a, 1109b and 1110 respectively denote electrodes. Reference numeral 1111 denotes an antireflection layer provided on an end facet of the device.